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5.2: Background

Difficulty Level: At Grade Created by: CK-12
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Ketones and aldehydes have very similar structures and therefore behave very similarly. Both contain a formyl group, or a carbon molecule double-bonded to an oxygen molecule. In ketones, the formyl group occurs somewhere in the middle of the hydrocarbon, while in aldehydes the formyl group occurs at the end. Both aldehydes and ketones are almost always polar because the oxygen pulls more of the electrons in the compound towards itself. However, when the molecule is so large that the pull of the oxygen is considered negligible, the molecule is nonpolar.

This is the diagram for all ketones. “R” represents the rest of the compound. Both R's do not have to be identical.

This is the diagram for all aldehydes. Again, the “R” represents the rest of the compound.

To remember the difference between aldehydes and ketones, let's take a look at the etymology of aldehyde. The name aldehyde comes from “alcohol dehydrogenated.” Recall from the chapter “Functional Group: Alcohol” that alcohols contain a hydroxyl group (-OH) on the end. If you remove the hydrogen, or dehydrate the alcohol, you obtain an aldehyde, not a ketone.

When naming aldehydes, start by counting the number of carbon atoms and give the compound the appropriate prefix. Then, add the suffix “–anal.” This is the molecule's IUPAC name. However, many aldehydes are more commonly referred to by another given name. For example, methanal is more commonly called formaldehyde.

(1) Hydrocarbon Base: methane (2) Prefix: meth- (3) IUPAC Name: methanal (4) Common Name: formaldehyde

(1) Hydrocarbon Base: propane (2) Prefix: prop- (3) IUPAC Name: propanone (4) Common Name: acetone

To name a ketone, first determine the name of the hydrocarbon it is based off of by counting the number of carbons. For example, if the compound has three carbons, its parent hydrocarbon is propane. Then, drop the “-e” from the parent hydrocarbon's name and add an “-one.” In this example, the ketone would be called “propanone.”

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